L5 - Autoscaling 2/2

Question 1

What are cooldown periods for policies?

Answer 1

• only after this time, breaches are handled again

Question 2

What does predictive autoscaling have to do with an autoscaling group?

Answer 2

It determines proactively the minimum of the autoscaling group and thus increases resources.

Question 3

What is a load balancer (LB)?

Answer 3

load balancer = single point of request and it then it is distributed across service instances

Question 4

When is LB important?

Answer 4

In the case of multiple service instances.

Question 5

Why do you need a LB to scale out?

Answer 5

Scaling out only works if all replicas are equally busy. LB is required to distribute requests.

Question 6

goals of LB?

Answer 6

• efficient utilization of a set of resources
• exploit aggregated capacity of replicas to reduce response time and failure rate
• increase availability (LB performs health checks, restarts faulty replicas/instances)
• enables non-disruptive management (in case of provisioning and de-provisioning of resources)

Question 7

Are there multi-layer LB?

Answer 7

Yes

Question 8

How is LB implemented?

Answer 8

• instances are allocated to certain VMs by the load balancer
• the VMs are load balanced to servers
• the LBs on different levels should interact with each other and exchange information

Question 9

What is the difference between static and dynamic LB?

Answer 9

• in static LB there is no feedback (e.g. weighted round robin)
• in dynamic LB there is feedback on the status of the servers

Question 10

Dynamic LB Diagram

Answer 10

Question 11

What are the two sub-categories of dynamic LB?

Answer 11

Distributed and non-distributed

Question 12

What are the two sub-categories of distributed LB?

Answer 12

Cooperative and non-cooperative

Question 13

What are the two sub-categories of non-distributed LB?

Answer 13

Central and semi-distributed

Question 14

What is distributed LB?

Answer 14

Nodes collaborate

Question 15

What is cooperative LB?

Answer 15

• nodes have the same goal (e.g. optimize memory)

Question 16

What is non-cooperative LB?

Answer 16

• nodes have different goals

Question 17

What is centralized LB?

Answer 17

• one central LB

Question 18

What is semi-distributed LB?

Answer 18

Nodes are partitioned and one LB is responsible for partition.

Question 19

4 approaches for web applications

Answer 19

• round-robin DNS
• DNS delegation
• Client-side random sampling
• Server-side load balancing

Question 20

Round-robin DNS

Answer 20

• domain name is mapped to multiple IP addresses
• IP addresses are given to clients in RR fashion

Question 21

name server

Answer 21

Name servers work as a directory that translates domain names into IP addresses.

Question 22

DNS delegation

Answer 22

• structure domain (e.g. tum.de) in two zones
• each zone has it’s own name server
• DNS request is forwarded to both zones
• the one resolving the address first wins the request

Question 23

What is DNS?

Answer 23

The Domain Name System (DNS) is the phonebook of the Internet. Humans access information online through domain names, like nytimes.com or espn.com. Web browsers interact through Internet Protocol (IP) addresses. DNS translates domain names to IP addresses so browsers can load Internet resources.

Question 24

Client-side random sampling

Answer 24

• client receives a list of IP addresses
• it selects randomly one to connect to

Question 25

Server-side load balancing

Answer 25

LB receives requests at a given port and distributes them

Question 26

What is session persistence in LB?

Answer 26

Session persistence ensures that a client will remain connected to the same server throughout a session or period of time. Because load balancing may, by default, send users to unique servers each time they connect, this can mean that complicated or repeated requests are slowed down.

stickiness/Session persistence = results in a “sticky session” between a user and a particular server. In this process, a load balancer uses logic to find an affinity between a specific network server and a client for the length of an entire session, defined by the amount of time a unique IP address stays on the site.

Question 27

How does a LB create sticky sessions?

Answer 27

A load balancer creates sticky sessions by either tracking a user’s IP details or using a cookie to assign that user an identifying attribute. This allows the load balancer to use the tracking ID to route all of that user’s requests to a specific server throughout the session.

Question 28

3 classes of LB algorithms

Answer 28

class-aware
content-aware
client-aware

Question 29

What is class-aware LB

Answer 29

based on classification of requests into the classes: sensitive, best-effort, undesired
eg. based on port numbers (e.g. port 1 is sensitive)

Question 30

What is content-aware LB?

Answer 30

based on request content e.g. URL
e.g. direcitng similar requests to the same server to exploit access to same information

Question 31

What is client-aware LB?

Answer 31

based on packet source
can improve performance as before

Question 32

4 LB algorithms

Answer 32

• Round Robin (RR) and Weighted Round Robin
• Least connection and weighted least connection
• resource based
• weighted response time

Question 33

Round Robin (RR) and Weighted Round Robin

Answer 33

RR is very simple and just allocates people to servers sequentially. Not good if some people use servers for a long time. → use smart load balancing.
Processors circularly assign each process without defining any priority. This results in a faster response in case of similar workload distribution among the processes. All the processes have different loading time. Therefore, some nodes might be heavily loaded, while the others may remain under-utilized
weight represents capability of server in weighted Round Robin

Question 34

Least connection and weighted least connection

Answer 34

distributes to server with the least number of active connections
Checks which servers have the fewest connections open at the time and sends traffic to those servers. This assumes all connections require roughly equal processing power.

Question 35

resource based LB algorithm

Answer 35

CPU load of the servers is taken into account
Distributes load based on what resources each server has available at the time. Specialized software (called an “agent”) running on each server measures that server’s available CPU and memory, and the load balancer queries the agent before distributing traffic to that server.

Question 36

weighted response time LB algorithm

Answer 36

the response time for a health check is used to compute the weights

Question 37

AWS Elastic Load Balancing (ELB)

Answer 37

• Distributes incoming traffic across the instances in the Auto Scaling Group
• Can use load balancer metrics (request counts per target) for auto scaling
• Can use it for health checks (elastic load balancer sends health check messages to instance to find out if they are active or not)

Question 38

Classic Load Balancer (old; 2009)

Answer 38

Distributes requests evenly across availability zones or evenly across all registered instances in the target group.

Question 39

Application Load Balancer (VPC only, Application Layer)

Answer 39

Routes http requests based on contents to specific target groups

Question 40

Network Load Balancer (VPC only, Transport Layer)

Answer 40

Forwards TCP packets for a certain port to a target group.

Question 41

Gateway Load Balancer (VPC only, Transport and Network Layer)

Answer 41

Forwards ingress traffic to network appliances, like intrusion detection or monitoring
Forwards response traffic from network appliances to target after inspection

Question 42

Hypertext Transfer Protocol (HTTP) (from internet)

Answer 42

HTTP is a protocol which fetches resources such as HTML documents. It is used for exchanging data on the Web and is a client-server protocol which means requests are initiated by the recipient usually the Web browser.
HTTP contains specific instructions on how to read and process this data once it arrives.
When you type a URL into your web browser, you are sending an HTTP request to a web server. That server will then respond, again using the formatting of HTTP.